1. Field of the Invention
This invention relates to a developing device for visualizing an electrostatic latent image formed on an image carrying member with a color material such as toner, and in particular to a developing device including a developer carrying member on which a thin film of developer such as toner is first formed, which is then applied to an electrostatic latent image to effectuate visualization of the latent image.
2. Description of the Prior Art
Developing devices using toner to develop the electrostatic latent image for use in various types of recording machines such as an electrophotographic copying machine and electrostatic recording machine are well known in the art. These developing devices may be generally categorized into two different systems: a single component developer system in which use is made of a single component developer comprised of magnetic toner particles and a duplex component developer system in which use is made of a duplex component developer comprised of toner particles and carrier beads. In either system, it is often required to form a film of developer having a controlled thickness, in particular an extremely thin film in some cases, before application to an electrostatic latent image so as to attain a desired developing performance.
The single component developer system has recently attracted a wide attention because of its potential capability in simplification in the overall device structure as well as enhanced reliability and performance in developing process as compared with the conventional duplex component developer system. In general, there are two developing methods in the single component developer system. One of them is the induction type developing method which uses toner particles of relatively low resistivity. In this method, when a thin film of toner particles is brought against an electrostatic latent image, charges opposite in polarity to the latent image are induced in those toner particles positioned opposite to the latent image and those toner particles charged oppositely by induction are then attracted to the latent image to effectuate development of the latent image.
The other developing method is the charging type developing method which uses toner particles of relatively high resistivity. In this method, a thin film of toner particles charged opposite in polarity to an electrostatic latent image to be developed is first formed and then brought into contact or proximity of the latent image to cause selective transfer of the toner particles to the latent image. This second method of charging type has numerous advantages and it has been expected to be able to produce a developed image of excellent quality which is almost as good as the one obtained by using a duplex component developer. However, an obstacle has existed in putting the charging type method in practical usage. That is, in order to attain an excellent developing performance, individual toner particles must be charged uniformly, and in order to attain such uniform charging, a film of toner particles must be made thinner, i.e., below the thickness of 4-5 particles stacked one on top of another or 50 microns or less for commonly sized toner particles. A major difficulty has resided in forming such a thin film of toner particles which is uniform in thickness as well as in charging.